The Counters are not specific to compilation units, they just happen to
be used in WasmCompilationUnit::ExecuteCompilation.
Remove it from the compilation unit and pass it explicitly where needed.
This saves another field on the compilation units.

R=titzer@chromium.org

Bug: v8:8343
Change-Id: Iad4fd8ae23b022c237535503e0e805db7e67071a
Reviewed-on: https://chromium-review.googlesource.com/c/1304297
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57083}